To investigate the possible role retrovirus onc-related genes in human neoplasia, we have analyzed human tumor cell lines for evidence of expression of onc-related genes. Our demonstration of simian sarcoma virus (SSV) sis-related transcripts in human fibrosarcoma cell lines, but not in their normal fibroblast counterparts, argues that the expression of this onc gene is probably associated with the transformed state of the cells. The human c-sis gene has been cloned into a vector designed to induce c-sis RNA expression. The biological effects of this DNA molecule are being examined by transfection analysis.